An x-ray source used in medical imaging systems, such as computed tomography systems, typically includes an evacuated x-ray envelope containing an anode and a cathode. X-rays are produced by applying a high voltage across the anode and cathode and accelerating electrons from the cathode against a focal spot on the anode. The x-rays diverge from the focal spot in a generally conical pattern.
Known cathode assemblies for such x-ray sources typically include a cathode cup, a cathode insulator, and several current carrying filaments. Each filament includes a coil and leads extending from respective ends of the coils. The cathode cup has a filament receiving portion for the filaments, and lead openings extend through the cup to the filament receiving portion. Cathode insulators are connected to the cathode cup, and the filament leads extend through respective lead openings and insulators to an energy source.
At least one known cathode insulator includes a hollow filament lead tube, a substantially cylindrical insulating member, and a nickel flange. A bore extends through the cylindrical insulating member and the filament is located in the insulating member bore. Particularly, the filament lead tube is positioned within the insulating member bore and is brazed to the insulating member so that the axis of the tube is coaxial with the axis of the insulating member. The tube extends from one end of the insulating member. The flange is brazed to an outer surface of the insulating member, and has a flange portion which extends radially outwardly from the insulating member outer surface.
To form the cathode assembly, the cathode insulator is spot welded to the cathode cup, and filaments are inserted into the cathode cup so that each filament coil rests in the filament receiving portion and each filament lead extends through a respective lead opening and cathode insulator. Particularly, and for each lead opening, the flange portion of the flange of an insulator is spot welded to the cathode cup so that the insulating member bore and filament lead tube are aligned with the respective lead opening. Filaments are then inserted into the cathode cup so that the filament leads extend through the lead openings and cathode insulators to a specific filament set height.
Precise positioning of the filaments is important because such positioning affects operation characteristics of the x-ray tube, such as focal spot size and position. Incorrect focal spot position causes image resolution loss and image degradation. Accordingly, it is desirable to properly position each filament lead, and particularly the filament coils, within the cathode cup.
To facilitate proper filament positioning, each filament lead, after being set to a predetermined filament set height, is flashed to its respective cathode insulator filament lead tube. The spot welding and flashing processes facilitate ensuring that each filament is positioned for correct focal spot positioning.
The welding and flashing processes described above, however, are time consuming and cumbersome. For example, flanges from adjacent cathode insulators often overlap, inhibiting satisfactory welding. Therefore, the flanges must often be trimmed before they are welded to the cathode cup. Trimming the flanges is a time consuming and tedious task. Furthermore, the welds between the respective flanges and the cathode cup may loosen during the flashing process and tube operation, causing the respective cathode insulators to move relative to the cathode cup. Such movement requires readjusting the filament, and re-flashing, and re-working the filament leads.
It would be desirable to eliminate flange trimming when attaching a cathode insulator to a cathode cup. It also would be desirable to eliminate the time consuming welding process and minimize the time consuming and costly process of repositioning, re-flashing, and re-working filaments within a cathode cup. Eliminating such processes, however, preferably would not result in any less precise positioning of the filament coils in the cathode cup.